802. 16j standard is considering an object called fixed relay station in addition to a base station and a mobile station of a Mobile Wimax system of the conventional 802. 16e standard, and advancing to expand a cell service coverage and to increase a system capacity using the fixed relay station. One of conditions suggested by International Telecommunication Union Radio communication Sector (ITU-R) as requirements of a candidate technology of International Mobile Telecommunication (IMT)-Advanced, is to provide the seamless handover to mobile stations moving fast at 120˜350 km. For the seamless and highly reliable communications of the mobile stations; that is, to support the mobility, the 802. 16j standard suggests and reflects a technique which reduces the handover delay in a manner that a mobile relay station is authorized by the mobile stations communicating with a serving base station to perform the handover to a target base station. According to this technique, the mobile relay station installed in a fast moving vehicle (e.g., bus or train) performs the handover in lieu of the mobile stations in the fast moving vehicle, and thus provides the seamless and highly reliable service to the mobile stations. Since the relay station can relatively freely adopt a multi-antenna technology compared to the mobile station even in the fast movement under severe fading, it is highly likely to overcome the fading. In terms of the power constraint, this technique is based on the hardware feature of the higher degree of freedom than the mobile station.
In the handover scheme of the mobile relay station of the 802. 16j standard, the mobile stations subordinate to the mobile relay station do not perform the handover and blindly operate using a CID mapping. That is, in the downlink, the target base station transmits a new CID of the mobile station, an old CID mapped to the new CID before the handover, and data destined for mobile station, to the mobile relay station. The mobile relay station forwards the data to the mobile station by changing the new CID of the MAC PDU destined for the mobile station with the old CID mapped to the new CID. In the uplink, the mobile relay station forwards data to the target base station by changing the old CID of the MAC PDU sent from the mobile station with the new CID mapped to the old CID. Accordingly, the mobile stations subordinate to the mobile relay station do not need to update the CID and can omit a security related procedure between the mobile station and the target base station. As such, as for the handover scheme of the mobile relay station in the 802. 16j standard, the mobile relay station represents its subordinate mobile stations, minimizes the message exchanges between the mobile stations and the mobile relay station, and carries out the handover through a first stage (the base station <−> the mobile relay station) message exchange between the mobile relay station and the base station, to thus minimize the handover delay.
A handover process in 802. 16 systems based on the Mobile Wimax is divided largely into a handover preparation procedure and a handover execution or action procedure. The handover execution or action procedure largely includes a synchronization acquisition procedure, a network re-entry procedure, and a connection set-up procedure. The handover scheme of the mobile relay station in the 802. 16j standard overlooks two important problems: one is the vulnerability in the security and the other is the residual handover delay according to the bandwidth request executed in the connection set-up procedure.
First, the security related problem is discussed. Basically, the handover standardization of the mobile Wimax based on the 802. 16e is considering the security related processing of the network re-entry procedure as the important requirement. This is because, in the handover, the security related processing at the level of the authorization of the mobile relay station is quite important in the malicious eavesdropping prevention and the e-mobile commercial business transaction processing. Hence, even a user boarding the fast moving vehicle is not exceptional in the security. However, in the handover of the 802. 16j system, the mobile stations subordinate to the mobile relay station do not need to update the CID and thus do not conduct the authentication procedure using Cipher-based Message Authentication Code (CMAC) of the base station. In conclusion, the handover of the 802. 16j system can minimize the handover delay using the CID mapping but is seriously vulnerable to the security related attack in the handover execution.
Next, the problem related to the bandwidth request of the connection set-up procedure is discussed. As stated above, the 802. 16j standard asserts that the handover is carried out through the handover related signaling of the first stage (the base station <−> the mobile relay station) between the mobile relay station and the base station and thus the handover delay time is minimized. However, for the mobile station to request the uplink resource allocation and to transmit an initial uplink packet to the target base station via the mobile relay station, the bandwidth request is inevitable. For the bandwidth request, a second stage (the base station <−> the relay station <−> the mobile station) handover related signaling is generated in the connection set-up procedure. That is, there still remains the handover delay time according to the connection set-up time. Evaluation Methodology Document (EMD) of the 802. 16m defines that the handover delay is the sum of the three procedures as mentioned above; that is, the sum of the time delays of the synchronization acquisition procedure, the network re-entry procedure, and the connection set-up procedure.
In short, the handover scheme of the mobile relay station according to the current 802. 16j standard does not take into account not only the security problem but also the connection set-up procedure including the bandwidth request. There is the trade-off between the handover delay problem which should stick to the first stage signaling and the security problem which should stick to the second stage signaling. Thus, what is needed is a method for minimizing the handover delay as in the 802. 16j handover scheme and for addressing the conflict with the security problem.